1. Field of the Invention
This invention relates to a magnetoresistance element for reading the magnetic field intensity of magnetic recording media as signals, especially a magnetoresistance element capable of reading a small magnetic field change as a greater electrical resistance change signal and a magnetic multilayer film suitable for use therein. The term "magnetoresistance" is often abbreviated as MR, hereinafter.
2. Prior Art
There are growing demands for increasing the sensitivity of magnetic sensors and increasing the density of magnetic recording. Active research works have been devoted for the development of magnetoresistance effect type magnetic sensors (simply referred to as MR sensors, hereinafter) and magnetoresistance effect type magnetic heads (simply referred to as MR heads, hereinafter). Both MR sensors and MR heads are designed to read out external magnetic field signals by detecting changes in the resistance of a reading sensors portion formed of magnetic material. The MR sensors have the advantage of high sensitivity and the MR heads have the advantage of high outputs in high density magnetic recording since the reproduced output does not depend on the relative speed of the sensors or heads to the recording medium.
Conventional MR sensors of magnetic materials such as Ni.sub.0.8 Fe.sub.0.2 (Permalloy) and NiCo utilizing anisotropic magnetoresistance effect offer only an MR ratio .DELTA.R/R as small as 2 to 5% and are low in sensitivity as reading MR beads adapted to accommodate for ultrahigh density recording of the order of several GBPI.
Attention is now paid to artificial superlattices having the structure in which thin films of metal having a thickness of an atomic diameter order are periodically stacked since their behavior is different from bulk metal. One of such artificial superlattices is a magnetic multilayer film having ferromagnetic metal thin films and antiferromagnetic metal thin films alternately deposited on a substrate. Heretofore known are magnetic multilayer films of iron-chromium and cobalt-copper types. Among them, the iron-chromium (Fe/Cr) type was reported to exhibit a magnetoresistance change in excess of 40% at liquid He temperature (4.2K) (see Phys. Rev. Lett., Vol. 61, page 2472, 1988). This artificial superlattice magnetic multilayer film, however, is not commercially applicable as such because the external magnetic field at which a maximum resistance change occurs (that is, operating magnetic field intensity) is as high as ten to several tens of kilooersted (kOe). Additionally, there have been proposed artificial superlattice magnetic multilayer films of Co/Ag, which require too high operating magnetic field intensity.
Under these circumstances, a three-element or ternary magnetic multilayer film having two magnetic layers having different coercive forces deposited through a non-magnetic layer was proposed as exhibiting a giant MR change due to induced ferrimagnetism. For example, European Patent Application No. 0483 373 proposing such a magnetic multilayer film in which two magnetic layers disposed adjacent to each other through a non-magnetic layer have different coercive forces (Hc) and a thickness of up to 200 .ANG.. Also the following reports are known.
(a) T. Shinjo and H. Yamamoto, Journal of the Physical Society of Japan, Vol. 59 (1990), page 3061
Co(30)/Cu(50)/NiFe(30)/Cu(50)!x15 wherein the number in the parentheses represents the thickness in angstrom of the associated layer and the number after "x" is the number of repetition (the same applies hereinafter) produced an MR ratio of 9.9% at an applied magnetic field of 3000 Oe and about 8.5% at 500 Oe.
(b) H. Yamamoto, Y. Okuyama, H. Dohnomae and T. Shinjo, Journal of Magnetism and Magnetic Materials, Vol. 99 (1991), page 243
In addition to (a), this article discusses the results of structural analysis, changes with temperature of MR ratio and resistivity, changes with the angle of external magnetic field, a minor loop of MR curve, dependency on stacking number, dependency on Cu layer thickness, and changes of magnetization curve.
(c) Hoshino, Hosoe, Jinpo, Kanda, Tsunashima and Uchiyama, Proceedings of Magnetics Research Meeting of the Japanese Electrical Society, MAG-91-161
This is a confirmation test of (a) and (b). Included are test of dependency on Cu layer thickness and dependency on NiFe layer thickness. Also reported is the result about the dependency of coercivity of Co on Cu layer thickness which is simulatively determined from the magnetization curve by extrapolation. Magnetization curves are derived from NiFe(30)-Cu(320) and Co(30)-Cu(320) and synthesized for comparison with a magnetization curve of NiFe(30)-Cu(160)-Co(30)-Cu(160). Since the thickness of the Cu intermediate layer is different from that of a three-element multilayer, direct comparison of squareness ratio and coercivity is impossible.
(d) Okuyama, Yamamoto and Shinjo, Proceedings of Magnetics Research Meeting of the Japanese Electrical Society, MAG-91-242
This article describes the phenomenological analysis on giant MR changes by induced ferrimagnetism. With the rotation of magnetic moment of an NiFe layer with low Hc, MR similarly changes. A giant MR phenomenon develops due to the artificially created spin anti-parallelism. It is proven by a difference in MR by an angular change of the applied magnetic field that this phenomenon is different from the anisotropic MR effect of NiFe or the like.
(e) H. Sakakima et al., Japanese Journal of Applied Physics, 31 (1992), L484
For RF sputtered NiFe/Co/Cu/Co multilayer film, micro-structure and MR ratio are examined. Reported is an oscillatory phenomenon of MR ratio with the thickness of Cu layer when both the NiFe and Co layers have a fixed thickness of 30 .ANG.. No magnetic field is applied during layer deposition.
(f) EP-A1 0483 373/1991
Disclosed is a magnetic multilayer film having two magnetic layers having different coercive forces stacked through an intervening non-magnetic layer. An exemplary structure includes a Ni-Fe layer of 25 .ANG. or 30 .ANG. thick, an intervening Cu layer, and a Co layer of 25 .ANG. or 30 .ANG. thick.
(g) JP-A 223306/1992
Disclosed is a magnetic multilayer film having two magnetic layers having different coercive forces stacked through an intervening non-magnetic layer. One magnetic layer is of CoPt base material.
These three-element magnetic multilayer films exhibit a giant MR ratio of about 10% under an applied magnetic field of up to about several hundreds of coersted, though the magnitude of MR ratio is small as compared with Fe/Cr, Co/Cu and Co/Ag. It is to be noted that these disclosures refer to only MR changes under an applied magnetic field of up to about 10 to 100 Oe.
For practical MR head material to find use in ultra-high density magnetic recording, an MR curve under an applied magnetic field of 0 to about 40 or 50 Oe is critical. The above-mentioned prior art three-element artificial superlattices, however, failed to increase MR changes under zero magnetic field, with an MR changes approximate to 0. An increase of MR change becomes maximum at about 60 Oe and an MR ratio of about 9% is then obtained. This implies that the MR curve has a slow rise. In the case of Permalloy (NiFe), the MR change has a gradient of approximately 0 across zero magnetic field, the MR ratio remains substantially unchanged, the differential value of MR ratio is close to 0, and magnetic field sensitivity is low. The material is not suitable as reading MR heads for ultra-high density magnetic recording.
For overcoming such properties, NiFe is provided with a shunt layer having low resistivity such as Ti for providing a shift of the operating point. In addition to the shunt layer, a soft film bias layer of soft magnetic material having a high resistivity such as CoZrMo and NiFeRh is provided for applying a bias magnetic field. The structure having such a bias layer, however, is complex in steps, difficult to provide stable properties, and increased in cost. It also invites a lowering of S/N since it uses a relatively moderate portion of the MR curve.
Moreover, MR heads have a complex layered structure, require heat treatment such as baking and curing of resist material during patterning and flattening steps, and must tolerate temperatures of about 350.degree. C. Conventional three-element artificial superlattice magnetic multilayer films, however, degrade their properties during such heat treatment.
The following reports were published after the filing date in Japan of the basic application, but before the filing date of this application in the U.S.
(h) EP-A1 0498 344/1992
Disclosed is a magnetic multilayer film having stacked through an intervening non-magnetic thin film two magnetic thin films of (Ni.sub.x Co.sub.1-x).sub.x' Fe.sub.1-x' and (Co.sub.y Ni.sub.1-y).sub.z Fe.sub.1-z wherein x=0.6-1.0, x'=0.7-1.0, y=0.4-1.0, and z=0.8-1.0. An exemplary structure includes a non-magnetic thin film of 50 .ANG. thick intervening between two magnetic thin films of 30 .ANG. thick.
(i) T. Valet et al., Applied Physics Letters, 61 (1992) 3187
For RF sputtered Ni.sub.80 Fe.sub.20 /Cu/Co multilayer film, an oscillatory phenomenon of MR ratio with the thickness of Cu layer is reported. Samples of NiFe(50)-Cu(x)-Co(20)-Cu(x)!x3 wherein 7.ltoreq.x.ltoreq.37 were deposed by RF sputtering in the absence of a magnetic field. It is described that an MR change component attributable to differential coercive force is found only at x=33 .ANG..
(j) T. Valet et al., Journal of Magnetism and Magnetic Materials, 121 (1993) 402
For RF sputtered Ni.sub.80 Fe.sub.20 /Cu/Co multilayer film, micro-structure and MR change are examined. Reported are cross-sectional electron photomicrographs (TEM) of two samples: NiFe(50)-Cu(50)-Co(10)-Cu(50)!x12 and NiFe(50)-Cu(50)-Co(100)-Cu(50)!x8, and MR characteristics of two samples: NiFe(50)-Cu(20)-Co(30)-Cu(20)!x18 and NiFe(50)-Cu(20)-Co(30)-Cu(20)!x3. A 18-layer stack, for example, shows an MR change of 11% at room temperature which is allegedly attributable to differential coercive force. No magnetic field is applied during layer deposition.
(k) Journal of Magnetism and Magnetic Materials, 121 (1993) 339
Described is a magnetization reversal mechanism of sputtered NiFe(50)-Cu(x)-Co(20)-Cu(x)!x3. MR characteristics are referred to nowhere. No magnetic field is applied during layer deposition.
In the aforementioned publications, no magnetic field is applied during layer deposition and NiFe layers are provided with no magnetic anisotropy and thus have high squareness ratios. As a result, the MR ratio in the range between -10 Oe and +10 Oe has great hysteresis centered at zero magnetic field and the MR slope in that range is small, indicating failure to provide satisfactory and stable reproduction as magnetic heads.
For MR heads intended for ultra-high density magnetic recording, an MR change curve under an applied magnetic field between -50 Oe and +50 Oe is important. However, in all the examples of the aforementioned publications, the MR ratio in the range between -50 Oe and +50 Oe has great hysteresis centered at zero magnetic field and the MR slope in that range is small.
Often MR heads are used in a high-frequency magnetic field of 1 MHz or higher for reproduction of high density recorded signals. Most prior art three-element magnetic multilayer films are difficult to provide high high-frequency sensitivity by producing an MR slope (or MR change curve slope) of 0.08%/Oe or more in a high-frequency magnetic field of 1 MHz or higher partly because of their film thickness combination.